Add stub_omniauth_setting

spec/support/stub_configuration.rb

@@ -25,6 +25,10 @@ module StubConfiguration
     allow(Gitlab.config.mattermost).to receive_messages(messages)
   end
 
+  def stub_omniauth_setting(messages)
+    allow(Gitlab.config.omniauth).to receive_messages(messages)
+  end
+
   private
 
   # Modifies stubbed messages to also stub possible predicate versions